Cooling device for the lubrication circuit of a compressor

ABSTRACT

In a cooling device for a rotary piston compressor, in particular a screw-type compressor, which is part of a refrigerant circuit together with a condenser and an evaporator, coolant of the refrigeration system and oil serving to lubricate bearings as well as to cool and seal the screw-type compressor are injected into the screw-type compressor. In order to cool the oil serving to lubricate the compressor bearings separately and to thereby achieve a more economical functioning of the entire cooling device, it is suggested that only the part of the oil serving to lubricate the bearings of the compressor is cooled as a function of the oil temperature sensed behind the bearings and that a branched stream of coolant is used for this, the stream on its part subsequently being fed to the compressor again.

The invention relates to a cooling device for a screw type compressor.

BACKGROUND OF THE INVENTION

Cooling devices of this type can be used in rotary piston compressorsfor refrigeration and air-conditioning systems, for example in ascrew-type compressor with oil injection. Refrigeration andair-conditioning systems essentially comprise an evaporator in whichheat is withdrawn from the environment by evaporation of the coolant, acompressor which increases the pressure of the vaporized coolant from asuction pressure to an outlet pressure, and a condenser in which thevaporized coolant under the discharge pressure is liquefied again underheat emission.

In screw-type compressors, two screw-like rotors meshing with each otherare arranged within the compressor casing for compressing the coolant,these rotors being tightly sealed radially by the compressor casing. Inmost cases, the screw-type compressors or worm compressors used inrefrigeration systems have a means for oil injection. The oil isinjected into the compression spaces of the screw-type compressors andthus into the gas to be compressed which is located therein. This servesessentially the following three purposes:

1. For cooling the compression process: By means of the injected oil,the coolant to be compressed is cooled and with that, the screw-typecompressor as a whole is also cooled. It is thereby subjected to smallertemperature differences. This means that fittings and tolerances can becarried out more closely, whereby the clearance losses in the compressorare decreased.

2. For lubricating the rotors and the bearings: Since in knownoil-injected screw-type compressors only one of the rotors is usuallydriven externally, for example by an electric motor or the like, theother rotor must be driven indirectly by and together with the drivenrotor. The injected oil thereby decreases the wear and tear at the tworotors. Besides this, the oil is used for lubricating the bearings ofthe rotors.

3. For sealing the clearances within the compression space: The injectedoil seals the clearances between the two rotors and between theindividual rotors and the compressor casing. In this manner, possiblyexisting leakage paths within the compressor are sealed and thus theconditions for a high degree of efficiency of the compressor arecreated. The oil injected into the compression chamber is vaporized andis carried along by the gaseous coolant to be compressed which islocated in the compression chamber. Thus, there is an oil-coolantmixture at the pressure outlet of the compressor. The oil found in theoil-coolant mixture must be separated from the coolant by oil separatorsin order to be injected again into the compressor and so that heattransmissions of the coolant within the refrigerating circuit are notinfluenced unfavourably.

When compressing to high pressures, the oil injected into the compressoris cooled as a function of a final temperature resulting at the pressureoutlet of the compressor. Thereby, cooling can take place by means ofcoolant injection, or by cooling with water or with air in a heatexchanger, e.g. a plate-type heat exchanger. A large quantity ofinjected oil necessitates large and expensive heat exchangers in thelatter case.

The temperature of the injected oil is essentially determined in thatits viscosity is great enough to ensure lubrication of the bearings.When the oil temperature increases, then the viscosity of the oildecreases and the lubrication of the bearings of the rotors isendangered. For the above-mentioned sealing of the clearances, whichrequires the greatest quantity of injected oil, also lower oilviscosities or higher oil temperatures, however, would be allowable.

SUMMARY OF THE INVENTION

The object of the invention is to controllably cool the oil used forlubricating the bearings in a simple and economical manner, independentof the total quantity of oil injected into the compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description of a preferred embodiment of the inventionserves to explain the invention in Greater detail in conjunction withthe attached drawing which schematically shows a cooling device.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As represented, a cooling device essentially comprises a screw-typecompressor 1, a condenser 2 and an evaporator 3 which are connected in aclosed refrigerant circuit by lines 4. Further, in the refrigerantcircuit is a check valve 5 which is arranged directly at the pressureoutlet of the compressor, an oil separator 6 which is arranged behindthe check valve 5 and in front of the condenser 2, as well as anexpansion member 7 which is located between the condenser 2 and theevaporator 3 in the refrigerant circuit.

Two temperature sensors 8 and 12 which cannot be seen individually arearranged in the compressor 1. A first temperature sensor 8 senses thetemperature at the bearings of the compressor 1 and is connected with acontrol unit 11 via an electric line 9. A second temperature sensor 12senses the temperature in the pressure outlet area of the compressor 1and is also connected with the control unit 11 via an electric line 13.

A main oil line 14 proceeds from the oil separator 6, the oil lineleading into the compression space of the compressor 1 via a solenoidvalve 15. From the main oil line 14, a bearing oil line 16 is branchedoff and leads into a heat exchanger 17 and from this to the bearings ofthe compressor 1.

Behind the condenser 2, a part of the coolant is branched off via a line18 from the line 4 of the refrigerant circuit, supplied to a solenoidvalve 20 controllable via an electric line 19 by the control unit 11 andfrom this valve via an injection nozzle 21 enters the heat exchanger 17from which it is supplied to a point 22 of the compressor 1 at which thesuction process of the compressor 1 activated by the rotors isconcluded.

The mode of operation of the cooling device is as follows: The coolantvaporized in the evaporator 3 is sucked in at the suction side of thecompressor 1 and is compressed therein. Oil is injected into thecompression space of the compressor via the main oil line 14 and thesolenoid valve 15. The oil is carried along by the coolant to becondensed and the oil-coolant mixture resulting in this manner issupplied in its condensed state to the oil separator 6 via the checkvalve 5. In the oil separator 6, the oil is separated from the coolantand, since it is under increased pressure, is injected again into thecompressor 1 via the main oil line 14 and the solenoid valve 15 at apoint of this compressor which is under lower pressure. The oil isseparated from the coolant so that the heat transmissions of the coolantwithin the refrigerant circuit are not adversely affected and, inaddition, to realize a closed main oil circuit.

When the temperature in the compressor 1 increases, which effects atemperature rise at the bearings of the rotors of the compressor 1, thenthe viscosity of the oil decreases, especially also the oil which isfound in the bearings. When a critical temperature at which theviscosity of the oil at the bearings has Greatly decreased, is exceeded,which is determined behind the bearings by the temperature sensor 8,then the solenoid valve 20 is opened by the control unit 11 via theelectric line 19 and coolant in liquid form is injected into the heatexchanger 17 via the line 18 and the injection nozzle 21. In the heatexchanger 17, the oil branched off from the main oil line 14 via thebearing oil line 16 for cooling the bearings is cooled by the coolantbranched off behind the condenser 2, whereby heat is supplied to thecoolant and heat is withdrawn from the oil used for lubricating thebearings. The coolant thereby vaporised in the heat exchanger 17 issupplied to the suction side of the compressor, advantageously at apoint 22 at which the suction process of the compressor 1 is concluded.The injection at this point 22 of the compressor 1 is necessary becauseotherwise the refrigeration capacity of the compressor, i.e. the amountof heat absorbed from the surroundings for vaporizing the coolant in theevaporator 3, decreases because the coolant branched off for cooling theoil serving as bearing lubricant does not contribute to the heattransmission in the evaporator 3. Furthermore, injecting the coolant atthe point 22 of the compressor 1 results in the advantage that thecoolant coming from the heat exchanger 17 meets with the partiallycompressed, warmer coolant in the compressor 1 and thereby cools downthe latter, which leads to an advantageous, lower final compressiontemperature.

Should the final compression temperature still exceed a predeterminedlimiting value, then the solenoid valve 20 is opened by means of thecontrol unit 11 via the electric line 13 with the second temperaturesensor 12 located in the pressure outlet area of the compressor 1 and bymeans of the injection nozzle 21, more coolant is injected into the heatexchanger 17 than is necessary for cooling the oil serving to lubricatethe bearings.

Should the final compression temperature increase further even when thesolenoid valve 20 is constantly open, then by means of the temperaturesensor 12 or by a further temperature sensor, not illustrated, in thepressure outlet area of the compressor 1, a switching off of thecompressor is achieved via the control unit 11.

The cooling of the oil used for bearing lubrication described aboveoffers the advantage of using oil of lower intrinsic viscosity.Previously, the demand for high intrinsic viscosity was determinedespecially by the lubrication of the bearings of the compressor, since asufficient operation viscosity of the oil at the bearings is necessaryat high bearing temperatures. On the "cold side" of the refrigerantcircuit, the use of oil having a high intrinsic viscosity could,however, cause problems. At low evaporation temperatures, the oil whichis not separated by the oil separator and, therefore, located in therefrigerant circuit becomes so viscous that it is no longer taken alongby the stream of gaseous coolant in the evaporator. In this manner, adisplacement of oil in the evaporator results, which can lead to adecreased heat transmission of the gaseous coolant, for example atevaporation pipes of the evaporator, or even lead to individual blockingof such pipes.

When, however, an oil of lower viscosity is used, then all the oilinjected into the compressor in known cooling devices has to be cooledin order to maintain the necessary operation viscosity at the bearings.Such cooling of all the injected oil is, however, subject to limitssince too great a decrease of the final compression temperature which ishereby effected, can lead to the fact that this comes close to thecondensation temperature and that coolant is already condensing in theoil separator. Besides this, the oil-coolant mixture has an essentiallylower viscosity than the pure oil and is no longer sufficient for thelubrication of the bearings since the coolant suddenly evaporates out ofthe oil at the warm bearings and in this manner interrupts thelubricating film at the bearings. The greater quantity of coolant in theoil also has disadvantages with respect to energy. This coolant must bejointly compressed resulting in higher energy requirements of thecompressor.

The main advantage of the controllable cooling of the oil forlubricating the compressor bearings according to the invention is inthat despite use of an oil of low intrinsic viscosity, a sufficientoperation viscosity of the oil serving to lubricate the bearings isachieved. Since the main oil stream located in the main oil line andprovided for injection into the compressor remains uncooled, the finalcompression temperature is prevented from sinking to critical valuesand, therefore, no coolant condenses into the oil in the oil separator.In addition, by means of the cooling of the bearing oil according to theinvention, the expense for cooling the oil is reduced considerably andthis increases the economic efficiency.

We claim:
 1. A cooling device for the bearings of a screw-typecompressor having a compression space and bearings and being connectedin a refrigerant circuit in which coolant is circulated through acondenser and an evaporator including in combination means including amain oil line for injecting oil into said compression space to becarried along by the coolant to be condensed to lubricate said bearingsand to seal the screw type compressor, means for diverting a branchedstream of coolant from said refrigeration circuit, a branch line fordiverting oil from said main oil line and feeding the diverted oil tosaid bearings, means for sensing the temperature of the oil at saidbearings and means responsive to said sensing means for cooling the oilin said branch line with said branched stream of coolant as a functionof the oil temperature sensed at said bearings.
 2. Device according toclaim 1, characterized in that the branched stream of coolant is fed tothe screw-type compressor (1) at a point (22) where the suction processof the screw-type compressor (1) is concluded.
 3. Device according toclaim 1, characterized in that the branched stream of coolant is fed tothe suction side of the screw-type compressor (1).
 4. Device accordingto claim 3 characterized in that the branched stream of coolant is fedto the screw-type compressor (1) at a point (22) where the suctionprocess of the screw-type compressor (1) is concluded.
 5. A coolingdevice as in claim 1 including means for controlling said branchedstream of coolant as a function of final compression temperature. 6.Device according to claim 5 characterized in that the branched stream ofcoolant is fed to the screw-type compressor (1) at a point (22) wherethe suction process of the screw-type compressor (1) is concluded. 7.Device according to claim 5, characterized in that the branched streamof coolant is fed to the suction side of the screw type compressor. 8.Device according to claim 7 characterized in that the branched stream ofcoolant is fed to the screw-type compressor (1) at a point (22) wherethe suction process of the screw-type compressor (1) is concluded.